More and more electronic applications are requiring distributed power architectures where the current requirements of the electrical loads are requiring the power supplies to be moved as close to the load as practicable. Instead of the single power supply which would accept ac line voltage and produce a dc or ac output voltage for use by an entire electrical system, today's ultra-fast electronics and electrical components often require their own power supply to accommodate the high transients in their load currents. This new concept in power systems is often referred to as a "distributed power architecture." This type of power architecture can be implemented by means of a system rectifier which converts the ac line current into an unregulated or slightly regulated dc voltage, and numerous "point-of-load" power supplies. The point-of-load power supplies accept the dc voltage from the rectifier and produce a highly regulated dc voltage which is able to accommodate very large current transients (large di/dt).
The point-of-load power supplies need to be small, have a high power density, and be mountable on the circuit boards near the load. In addition, the point-of-load power supplies should be modular to allow two or more to be connected in parallel to supply power to high current loads. This modularity allows a single design to be adapted for loads with varying current requirements. The problem with placing power supply modules in parallel is getting the modules to share current effectively.
Small variances in component values or reference levels will cause one or two paralleled power supplies to supply the majority of load current while some of the remaining modules supply relatively little, or no, current. This disparity in load currents causes the modules supplying the majority of the current to wear faster due to the increased thermal stresses, leading to premature failures in the field. Accordingly, several methods have been tried to force parallel power supply modules to share load current evenly.
One prior method of current sharing was to replace the internal error reference voltages of each module, which control the duty cycles of the power supply module's switches, with a single system reference voltage generated by a common error amplifier. The disadvantages of this system are that it provides a single point of failure in that if the common error amplifier fails, then the entire power system fails. Additionally, it can be problematic to ship the bus error voltage to all of the power supply modules, and differences in the bus error voltage can appear at each module due to differing physical distances from the common error amplifier to the individual power supply modules.
Another method of current sharing involves master/slave schemes where the "master" module provides the control information used by each of the "slave" modules. These systems can provide adequate current sharing, but suffer from drawbacks. First, it requires two different types of power supply modules, a master module and a slave module. This reduces the advantages of modularity when two different parts must be used in manufacturing and repair. Next, there is again a single point of failure. If the master module fails the entire system is brought down because the slave modules cannot generate their own control signals. Additionally, the master/slave arrangement requires both an error voltage bus as well as a synchronization bus to synchronize the internal clocks of each module.
Accordingly, what is needed is an improved method and circuit to provide for current sharing between parallel power supply modules, and modular power supplies which incorporate this improvement.